1. Field of the Invention
The present invention relates to an image reading apparatus and, more particularly, to an image reading apparatus suitable for an apparatus such as a flatbed image scanner or digital copying machine, which reads the image information of an original using an integrated scanning optical system unit in which a light source, a plurality of mirrors, imaging lens, image sensor, and the like are integrally accommodated.
2. Related Background Art
Conventionally, various image reading apparatuses such as flatbed image scanners or digital copying machines have been proposed.
FIG. 1 is a view schematically showing the arrangement of the main part of a conventional image reading apparatus of this type. Referring to FIG. 1, an original 103 placed on an original glass table 104 is illuminated with direct light from a light source 106 and a light beam that has passed through a reflector 107 from both sides. The image of the light beam reflected by the original 103 is formed on an image sensor 114 such as a CCD by an imaging lens 113 through a first mirror 108, second mirror 109, and third mirror 110 for scanning, and is converted into an electrical signal in accordance with a level light and shade (density) of the original 103. With this operation, image information of one line in the main scanning direction (direction perpendicular to the drawing sheet of FIG. 1) is read. Image reading in the sub-scanning direction (direction indicated by an arrow C in FIG. 1) is done in the following way. The first mirror table formed from the light source 106, reflector 107, and first mirror 108 is moved relative to the original 103 in the sub-scanning direction, and the second mirror table formed from the second mirror 109 and third mirror 110 is moved in the same direction at a speed 1/2 that of the first mirror table. With this operation, the image information of the original is read while maintaining a constant optical path length between the original 103 and image sensor 114.
The scanning optical system of this type is called a so-called 1:2 scanning optical system as a known technique. Even when the optical path length between the original and image sensor is increased, the apparatus size in the sub-scanning direction can be reduced.
However, since an original must be scanned while changing the relative positional relationship between the reading section comprising the imaging lens and image sensor that are fixed with respect to the original and the three scanning mirrors, color registration misalignment, density level variation, and defocus may occur due to vibration, angle error, and surface precision error of the scanning mirrors. To prevent this, the component precision or driving accuracy of the scanning mirrors must be increased.
FIGS. 2 and 3 are views schematically showing the arrangements of the main part of image reading apparatuses having an integrated scanning optical system unit, which relax the problem of the above 1:2 scanning optical system. Each of the integrated scanning optical system units shown in FIGS. 2 and 3 integrally accommodates components including a light source, reflector, a plurality of scanning mirrors, imaging lens, and image sensor, so an original is scanned without changing the relative positional relationship between the components.
FIG. 2 is a view schematically showing the arrangement of the main part of an image reading apparatus having an integrated scanning optical system unit disclosed in, e.g., Japanese Patent Application Laid-Open No. 63-217872. An integrated scanning optical system unit 118 shown in FIG. 2 integrally accommodates a light source 106 and reflector 107 for illuminating an original 103 placed on an original glass table 104, image sensor 114 for reading a light beam from the original 103 illuminated with the light source 106 and reflector 107, a plurality of scanning mirrors 115, 116, and 117 for guiding the light beam from the original 103, and imaging lens 113 for forming the image of the light beam from the original 103, which is based on image information, on the image sensor 114.
Referring to FIG. 2, the plurality of scanning mirrors comprise the first mirror 115, second mirror 116, and third mirror 117. A light beam from the original 103 travels from the first mirror 115 to the second mirror 116 and then from the second mirror 116 to the third mirror 117. The light beam from the third mirror 117 strikes the second mirror 116 again. After this, the light beam passes between the first mirror 115 and third mirror 117 and enters the imaging lens 113 to form its image on the image sensor 114. A plane connecting the second mirror 116, imaging lens 113, and image sensor 114 is substantially parallel to the original surface.
FIG. 3 is a view schematically showing the arrangement of the main part of an image reading apparatus having an integrated scanning optical system unit disclosed in, e.g., Japanese Patent Application Laid-Open No. 9-69915. An integrated scanning optical system unit 122 shown in FIG. 3 integrally accommodates a light source 106 and reflector 107 for illuminating an original 103 placed on an original glass table 104, image sensor 114 for reading a light beam from the original illuminated with the light source 106 and reflector 107, a plurality of scanning mirrors 119, 120, and 121 for guiding the light beam from the original 103, and imaging lens 113 for forming the image of the light beam from the original 103, which is based on image information, on the image sensor 114.
Referring to FIG. 3, illumination light by the light source 106 and reflector 107 illuminates the lower surface of the original 103 and is diffused and reflected. Some components of the light beam travel downward in the vertical direction in FIG. 3, are reflected by the first mirror 119, and reach the second mirror 120. The light beam incident on the second mirror 120 is reflected at a predetermined angle and strikes the first mirror 119 again. The light beam incident on the first mirror 119 again is further reflected at a predetermined angle. The light beam is reflected by the third mirror 121 in the horizontal direction and is incident on the imaging lens 113. A reduced image of the original 103 is formed on the image sensor 114 through the imaging lens 113, thereby reading the image information of the original 103.
These conventional image reading apparatuses have various problems to be described below.
In Japanese Patent Application Laid-Open No. 63-217872 shown in FIG. 2, the optical path from the original 103 to the first mirror 115 and that from the second mirror 116 to the imaging lens 113 cross each other, and additionally, the light beam is reflected by the second mirror 116 twice. For this reason, a space is required between the second mirror 116 and imaging lens 113, and consequently, the distance from the second mirror 116 to the image sensor 114 increases, resulting in a bulky integrated scanning optical system unit 118. Accordingly, the image reading apparatus using the integrated scanning optical system unit also becomes bulky.
In addition, the light beam from the second mirror 116 passes between the first mirror 115 and third mirror 117 and enters the imaging lens 113. With this arrangement, light components other than those contributing to image formation may be irregularly reflected by the edge portion of the first mirror 115 or third mirror 117 and enter the imaging lens as hazardous light. This generates ghost or flare to degrade the read image.
In Japanese Patent Application Laid-Open No. 9-69915 shown in FIG. 3, the third mirror 121, imaging lens 113, and image sensor 114 are parallel to the original surface and disposed immediately under the original glass table 104. The light source 106 and imaging lens 113 must therefore be arranged close to each other. In this arrangement, the light beam emitted from the light source 106 may directly enter the imaging lens 113, resulting in ghost or flare and degrading the read image.
In addition, since the optical path from the original 103 to the first mirror 119 and that from the third mirror 121 to the imaging lens 113 cross each other, a space is required between the third mirror 121 and imaging lens 113. Consequently, the distance from the third mirror 121 to the image sensor 114 increases, resulting in a bulky integrated scanning optical system unit 122. Accordingly, the image reading apparatus using the integrated scanning optical system unit also becomes bulky.
Furthermore, a read position 123 is located on the left end side of the integrated scanning optical system unit 122 with respect to a sub-scanning-direction width D of the integrated scanning optical system unit 122. For this reason, when the original 103 is scanned from the left end to the right end, a more space is necessary outside the read region at the right end, resulting in an image reading apparatus with an unbalanced arrangement.
Also, since mirrors having almost the same size as the original size in the main scanning direction are inserted on the lower side of the integrated scanning optical system unit, a space having the same size as that shown in the sectional view of FIG. 3 is required at any position in the main scanning direction. As a result, the space on both sides of the imaging lens in the main scanning direction cannot be effectively used.